Ultrasonic method for material monitoring

ABSTRACT

An ultrasonic measurement method in an ultrasonic measurement system having an ultrasonic transducer includes emitting an ultrasonic pulse from the ultrasonic transducer. An ultrasonic pulse is received in accordance with a pulse travel time wherein the amplitude of the received ultrasonic pulse varies according to the pulse travel time. A first electrical signal is provided representative of the received ultrasonic pulse wherein the amplitude of the first electrical signal varies in accordance with the pulse travel time. A second electrical signal is provided in accordance with the first electrical signal wherein the amplitude of the second electrical signal is substantially independent of the pulse travel time. In order to determine the second electrical signal variable amplification is applied to the first electrical signal in accordance with the travel time. A distance is determined according to the pulse travel time wherein the distance is representative of the distance between the transducer and a material surface.

This application is a continuation of application Ser. No. 08/653,623filed May 24, 1996, now U.S. Pat. No. 5,760,309.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to measurement systems for monitoringvarious parameters. In particular the present invention relates toultrasonic parameter measurement systems that operate by emitting anddetecting acoustic waves to determine information representing themonitored parameters.

2. Background Art

Ultrasonic instruments have been used to monitor the parameters ofmaterials in a large variety of measuring applications. When the levelor height of the surface of a material from the bottom of a container isthe parameter to be measured, an ultrasonic instrument can include atransducer for emitting an ultrasonic pulse in the direction of thematerial and detecting the echo of the ultrasonic pulse reflected fromthe surface of the material.

The time interval between the emission of the ultrasonic pulse and thedetection of the echo of the ultrasonic pulse is determined by thedistance between the transducer and the surface of the material. Thetime interval is measured by the ultrasonic instrument and the measuredtime is multiplied by the speed of sound to calculate the round tripdistance traveled by the ultrasonic pulse. The product of themultiplication can be divided by two to yield the separation between thetransducer and the surface of the material and otherwise scaled asdesired.

In practice ultrasonic measurement can be complicated by many factors.For example, the speed of sound through a medium is not a constant. Thespeed of sound varies with the temperature and the composition of themedium through which the ultrasonic pulse travels. Compensating forfactors that alter the speed of sound is well known. However, even ifthe factors altering the speed of sound are not compensated, the effectsof the factors are predictable and an uncompensated measurement can beconsidered approximately correct.

Additionally, it can be difficult to determine when an echo of atransmitted ultrasonic pulse is received by an ultrasonic measurementinstrument. Received signals interpreted as a reflected pulse from thematerial surface by an instrument can be caused by something else in themeasuring environment. This can lead to an instrument output that is inerror. Furthermore, a received signal can include echo signals returnedby a variety of paths. The timing and strength of the various receivedsignals depend upon the path traveled and the materials encounteredalong the path.

For instance, a received signal can contain an echo from the materialsurface, or an echo from the bottom of the vessel containing nomaterial, and an echo from an object inside the vessel, such as a pipe.When an ultrasonic measurement instrument receives a strong echo from,for example, a pipe located above the material surface, it can generatean output indicating that the material level is at the level of thepipe. The effects of errors of this nature can be serious. For example,when hazardous materials are involved a critical control action can beundertaken or not undertaken in reliance upon an incorrect indication ofmaterial condition.

One method of addressing some of the problems associated with ultrasonicinstrument measurement is profiling. In the profiling process, thevessel containing the material to be measured is emptied. Ultrasonicpulses are emitted into the vessel and the echo signals are received.All echo signals received are stored. The stored information can laterbe used to cancel the spurious echoes.

While the profiling measurement method can improve measurement accuracy,it has several drawbacks. There is substantial expense associated withthe electronic systems necessary to acquire, store and compare the data.Furthermore, profiling is cumbersome and time consuming. It is alsosubject to errors from changes in the measuring environment, such aschanges in the material properties or modification of the vessel or itsinternal apparatus. Any such changes may require that the profiling berepeated.

It is therefore a general object of the present invention to provide anultrasonic measuring system that avoids the drawbacks of existingultrasonic measuring systems.

It is another object of the present invention to provide an ultrasonicmeasuring system that is easily calibrated.

It is another object of the present invention to provide an ultrasonicmeasuring system with improved measurement accuracy.

It is another object of the present invention to provide an ultrasonicmeasuring system that is adaptable for use in standard instrumentationsignaling and power systems, including two-wire systems and digitalsignaling systems.

It is another object of the present invention to provide an ultrasonicmeasurement system which is simple, rugged, reliable and inexpensive.

In accordance with the foregoing objects, the present invention includesa novel system for processing received ultrasonic signals that improvesthe accuracy of determining when a received signal corresponds to amaterial condition of interest. The system of the present inventionincludes means for varying the responsiveness of the received signalprocessing circuitry as a function of the elapsed time after anultrasonic pulse is transmitted.

Other objects and features of the present invention will be understoodwith reference to the drawings, the following description and theappended claims.

SUMMARY OF THE INVENTION

An ultrasonic measurement method in an ultrasonic measurement systemhaving an ultrasonic transducer includes emitting an ultrasonic pulsefrom the ultrasonic transducer. An ultrasonic pulse is received inaccordance with a pulse travel time wherein the amplitude of thereceived ultrasonic pulse varies according to the pulse travel time. Afirst electrical signal is provided representative of the receivedultrasonic pulse wherein the amplitude of the first electrical signalvaries in accordance with the pulse travel time. A second electricalsignal is provided in accordance with the first electrical signalwherein the amplitude of the second electrical signal is substantiallyindependent of the pulse travel time. In order to determine the secondelectrical signal variable amplification is applied to the firstelectrical signal in accordance with the travel time. A distance isdetermined according to the pulse travel time wherein the distance isrepresentative of the distance between the transducer and a materialsurface.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram illustrating an ultrasonic measuringenvironment in which the present invention can be used.

FIG. 2 is a block diagram illustrating a prior art ultrasonicmeasurement instrument.

FIG. 3 is a block diagram illustrating the ultrasonic measurementinstrument of the present invention wherein a high discriminationvariable amplification device is controlled by a microprocessor.

FIG. 4 is a graphical representation of signal strength characteristicswhich can occur in an ultrasonic measurement instrument such as theultrasonic measurement instrument of FIG. 3.

FIG. 5 is a block diagram representation of an integrator for control ofa variable amplification device that can be used in an alternateembodiment of the ultrasonic echo signal processor portion of theultrasonic measurement instrument of FIG. 3.

FIGS. 6A, B, C are schematic representations of an ultrasonic echosignal processor suitable for use with the ultrasonic measurementinstrument of FIG. 3.

FIG. 7 is a block diagram representation of an alternate embodiment ofthe ultrasonic measurement instrument of FIG. 3 having a parametricsignal processor and a parametric generator.

FIG. 8 is a block diagram representation of an alternate embodiment ofthe echo signal processor of the ultrasonic measurement instrument ofFIG. 3.

FIG. 9 is a block diagram representation of an alternate embodiment ofthe ultrasonic measurement system of FIG. 3 having dual ultrasonictransducers driven in parallel.

FIG. 10 is a block diagram representation of an alternate embodiment ofthe ultrasonic measurement system of FIG. 3 having dual ultrasonictransducers that are alternately driven.

FIG. 11 is a block diagram representation of a system suitable for usewith the ultrasonic measurement system of the present invention.

FIG. 12 is a block diagram representation of a system suitable for usewith the ultrasonic measurement system of the present invention.

FIG. 13 is a block diagram representing an alternate embodiment of theultrasonic measurement system of the present invention having monitoringinput.

FIG. 14 is a block diagram representation of an alternate embodiment ofthe ultrasonic measurement system of the present invention.

FIGS. 15A, B are curves representing tone bursts from an ultrasoniccrystal.

DETAILED DESCRIPTION OF THE INVENTION

Referring now to FIG. 1, there is shown a schematic diagram illustratingultrasonic measurement system 30 for making an ultrasonic measurement.An ultrasonic measurement can be made within ultrasonic measurementsystem 30 either in accordance with the known prior art or in accordancewith the system of the present invention. Within ultrasonic measurementsystem 30 storage tank 12 or storage vessel 12 is used to store material14 having a material level L. The material level L of material 14 withinstorage vessel 12 can be the parameter of interest for measurementsperformed using ultrasonic measurement system 30.

Ultrasonic instrument system 2 is provided within ultrasonic measurementsystem 30 for determining an output signal on output line 28. The outputsignal is representative of material level L within storage vessel 12.Instrument system 2 is an ultrasonic instrument system that comprises asensor 4 and a electrical circuitry 10 coupled to sensor 4, such as bytelemonitoring cable 8, in order to permit remote monitoring ofelectrical circuitry 10. Sensor 4 contains an electroacoustic transducer6 that is typically an ultrasonic crystal and may be made of bariumtitanate or lead zirconate or any other material known to be used toform ultrasonic crystals. An ultrasonic crystal for this type ofmeasurement can have a resonant frequency of fifty kilohertz.

Electrical circuitry 10 of instrument system 2 applies an electricalsignal to electroacoustic transducer 6 by way of telemonitoring cable 8.The electrical signal applied by electrical circuitry 10 is a short ACburst at a frequency at or near the resonant frequency ofelectroacoustic transducer 6. The peak voltage of the burst fromelectrical circuitry 10 can be several hundred volts. Electroacoustictransducer 6 converts the applied electrical signal from electricalcircuitry 10 into an acoustic signal. The acoustic signal thus generatedis launched into the interior of storage vessel 12 by electroacoustictransducer 6 in the direction of material 14 as ultrasonic pulse 26.

Material 14 is covered by further material 16. Further material 16 canbe air, although it can be any other gaseous or liquid material.Ultrasonic pulse 26 travels through further material 16 or air 16 abovematerial 14 and encounters material surface 18 at the interface betweenair 16 and material 14. At surface 18 of material 14 a reflection ofultrasonic pulse 26 occurs due to the change in the speed of sound atthe interface Reflected ultrasonic pulse 26 travels back through air 16and is returned to transducer 6 as an ultrasonic echo signal or anultrasonic return signal.

Electroacoustic transducer 6 or ultrasonic transducer 6 converts theultrasonic return signal reflected from material surface 18 into anelectrical signal. The electrical signal from transducer 6 is applied toelectrical circuitry 10 by way of cable 8. Electrical circuitry 10detects the echo-related electrical signal from transducer 6 anddetermines time T. Time T is the round trip travel time that elapsesbetween the applying of the transducer-energizing electrical signal totransducer 6 and the receiving of the ultrasonic return signal. Becauseultrasonic pulse 26 travels distance D between transducer 6 and materialsurface 18 twice, distance D can be calculated by electrical circuitry10 from travel time T and the speed of sound R as D=RT/2, wherein theinverse speed of sound can be approximately 1.77 milliseconds/foot.

As previously described, the output of electrical circuitry 10 isprovided at output line 28. The signal on output line 28 of electricalcircuitry 10 can represent level L of material surface 18. Level L canbe calculated by electrical circuitry 10 by subtracting the calculateddistance D from the distance of ultrasonic transducer 6 from vesselbottom 24. In alternate embodiments of ultrasonic measurement system 30other scaling operations can be performed to generate other suitable anduseful signals on output line 28 from measured travel time T.

Echo signals reflected from material surface 18 that relate to theparameter of interest are not the only acoustic signals applied totransducer 6 as a result of ultrasonic pulse 26. Echoes from otherobjects or conditions in the measuring environment are also returned toultrasonic transducer 6 by a variety of paths as a result of spuriousreflections. The spurious reflections cause spurious electrical signalswithin electrical circuitry 10 that complicate the measurement ofdistance D within storage vessel 12.

Additionally, objects within storage vessel 12 such as pipe 20 andagitator 22 can cause spurious reflections of ultrasonic pulse 26. Thestrength and timing of the spurious reflections of ultrasonic pulse 26can vary widely. When surface level 18 is below the reflecting object,the strength and timing can be generally constant. However, whenmaterial surface 18 is above the reflecting object, the strength andtiming of the reflected signal can vary with distance D. Discontinuitiesin vessel walls 31 can also cause reflections with similar properties.

Additionally, vessel walls 31 of storage vessel 12 can also causespurious echoes. Furthermore, the spurious echoes caused by vessel walls31 can occur by way of paths involving multiple reflections. An exampleof such an echo path is the path taken by an ultrasonic pulse 26reflected from agitator 22, bouncing off vessel walls 30 and returningto transducer 6.

Referring now to FIG. 2, there is shown a block diagram representationof prior art ultrasonic measurement system 40 having ultrasonic crystal50 for performing ultrasonic measurements within ultrasonic measurementsystem 30. Ultrasonic measurement system 40 operates under the controlof microprocessor 81. In order to initiate ultrasonic pulse 26 fromultrasonic crystal 50, microprocessor 81 applies electrical crystalexcitation signal 83 including a pulse to excitation line 42. Crystalexcitation line 42 is coupled to transducer driver 44 and crystalexcitation signal 83 is thus applied to transducer driver 44 by way ofcrystal excitation line 42. Crystal excitation signal 83 can be a smallduty cycle signal wherein the pulse of crystal excitation signal 83 canhave a duration of approximately one-hundred microseconds. Crystalexcitation signal 83 can have a repetition period of approximatelyone-hundred to one-hundred forty milliseconds.

A counter or timer is started by microprocessor 81 when microprocessor81 applies crystal excitation signal 83 to crystal excitation line 42.The timer is used by microprocessor 81 to determine the total traveltime of ultrasonic pulse 26. In response to crystal excitation signal 83of crystal excitation line 42 transducer. Driver 44 generates anultrasonic AC signal pulse or burst for driving ultrasonic crystal 50 ofultrasonic transducer 6.

A high voltage is induced on the secondary of transformer 46 because ofthe sharp change in the voltage level of the pulses within excitationsignal 83 that are applied to the primary of transformer 46 bymicroprocessor 81. For this reason the ultrasonic burst can have avoltage amplitude between three hundred volts and four hundred fiftyvolts peak-to-peak at the secondary of transformer 46. The frequency ofthe ultrasonic burst can be fifty kilohertz. However, it will beunderstood that any acceptable frequency range and voltage amplitude canbe used for driving ultrasonic crystal 50.

The burst signal on the secondary of transformer 46 is applied as adrive pulse to crystal 50 which is part of ultrasonic sensor 52. Thefifty kilohertz drive pulse is applied to ultrasonic crystal 50 by wayof diodes 48. Diodes 48 conduct the high voltage drive pulse fromtransformer 46 but are nonconductive after the high voltage drive pulsehas decayed. Diodes 48 thus prevent transformer 46 from loading down theecho-responsive signals from ultrasonic crystal 50. In an alternateembodiment of ultrasonic measurement system 40 with an LC circuit (notshown) tuned to the frequency of ultrasonic crystal 50 can be provided.

When ultrasonic pulse 26 reflects from a surface such as materialsurface 18 and is applied to transducer 6 it is converted intoelectrical signals by crystal 50. The electrical signals formed bycrystal 50 in this manner can be in the millivolt, or even microvolt,range. They are applied to the input of echo signal processor 58 by wayof a voltage limiting circuit that includes resistor 54 and diodes 56.The voltage limiting circuit protects echo signal processor 58 from thehigh voltage drive pulse of the secondary of transformer 46 used toexcite crystal 50 during the initiation of ultrasonic pulse 26. Resistor44 limits the current applied to echo signal processor 58 and diodes 56clamp the voltage applied to echo signal processor 58 to approximatelyseven tenths of a volt during the initiation of ultrasonic pulse 26,while permitting the much smaller echo-representing signals from crystal50 to pass.

Echo signal processor 58 processes the echo-representing signals fromultrasonic crystal 50 and generates detect signal 69 on detect line 71in response to the echo-representing signals. Detect signal 69 isrepresentative of the distance D between ultrasonic transducer 6 andmaterial surface 18. The operations of echo signal processor 58 fordetermining detect signal 69 can be implemented using a variety ofanalog and digital functional blocks and a variety of different types ofcircuitry well understood by those skilled in the art.

Echo signal processor 58 of ultrasonic measurement system 40 can includeinput amplifier 60, switched attenuator 62, amplifier 64 and detector 68for processing the electrical signals representing ultrasonic pulse 26.It will be understood by those skilled in the art that ultrasonic pulse26 generated by transducer 6 is not an ideal signal. It requires asubstantial rise time in order to reach its peak amplitude as excitationenergy is applied to ultrasonic crystal 50 by transformer 46.Additionally, ultrasonic crystal 50 has a decay or a ring down time asstored energy is released from it after excitation of crystal 50 andafter the peak response of crystal 50. The ring down signal ofultrasonic crystal 50 can decay exponentially.

Therefore, in the preferred embodiment of echo signal processor 58 inputamplifier 60 is adapted to provide low noise high gain to the smallsignal received from transducer 6 prior to processing by switchedattenuator 62. Switched attenuator 62 is adapted to overcome the effectof the ringing down of ultrasonic crystal 50 during measurementsperformed by ultrasonic measurement device 40. Amplifier 64 is adaptedto further amplify the signal provided by transducer 6, and to perform avoltage shift. These operations make detect signal 69 suitable forprocessing by conventional logic circuitry.

An echo signal reflected from material surface 18 close to transducer 6has a relatively high amplitude relative to the ring down signal ofultrasonic crystal 50. Thus, if the amplitude of the entire signalreceived by transducer 6 is lowered by switched attenuator 62, the ringdown signal of crystal 50 can be effectively eliminated while the echosignal can remain for processing by switched attenuator 62 and amplifier64. Therefore, switched attenuator 62 can provide relatively smallamplification beginning when excitation pulse 83 initiates excitation ofcrystal 50, and continue providing relatively small amplification untilringing down has ended. After that period gain provided by switchedattenuator 62 can begin to rise. This process is sometimes referred toas time varying gain.

The ringing down problem can also be solved by using a better ultrasoniccrystal 50 having a response more closely resembling an ideal response.However, ultrasonic crystals of this quality are too expensive for mostultrasonic measurement applications.

Detector 68 of echo signal processor 58 compares the output of amplifier64 with the threshold input provided by threshold control circuit 70.The comparison by detector 68 produces detect signal 69 on detect line71. Detector 68 squares up the input it receives to provide an outputsuitable for processing by conventional logic circuits. Thus, detector68 functions, effectively, as an analog-to-digital converter. Detectsignal 69 is applied to the interrupt of microprocessor 81 by way ofdetect line 71 when the processed echo signal level exceeds thethreshold input determined by threshold control circuit 70. In general,relatively lower thresholds are preferred to relatively higherthresholds when adjusting threshold control circuit 70. However, if thethreshold of circuit 70 is too low echo signal processor 58 becomes toosensitive to noise. A threshold value of 1.3 volts has been found to besuitable.

As previously described, microprocessor 81 includes a timer that isstarted when crystal excitation signal 83 is applied on crystalexcitation line 42. The timer is stopped by microprocessor 83 whendetect signal 69 is received from detector 68 of echo signal processor58 by microprocessor 81. Thus excitation signal 83 and detect signal 69are used by microprocessor 81 to determine time T between the initiationof ultrasonic pulse 26 and the receiving of a corresponding ultrasonicecho signal. Time T determined in this manner by microprocessor 81 thusrepresents the round-trip travel time of ultrasonic pulse 26.

In the preferred embodiment of the present invention microprocessor 81also receives inputs from a variety of input control circuits. The inputcontrol circuits can be input control switches in the preferredembodiment of the invention. The input control switches ofmicroprocessor 81 can include operating mode switches 72. Operating modeswitches 72 can select from various modes of operation of ultrasonicmeasurement system 40.

For example, operating mode switches 72 can permit selection of modessuch as high level fail safe or low level fail safe, setting measuringunits to English or metric, providing time delay and selecting amongcalibration modes. The control switches coupled to microprocessor 81 canalso include calibration switches 74. Calibration switches 74 can beused for setting the zero values and the span or full scale values ofstorage vessel 12. Alarm switches 76, also coupled to microprocessor 81,can be used for setting levels within storage vessel 12 at whichultrasonic measurement system 40 generates alarm outputs.

Sensor 52 of ultrasonic measurement system 40 can include a temperaturesensor 78 for measuring the temperature in the measuring environment ofultrasonic transducer 6. Temperature sensor A/D converter 80 can also beincluded within ultrasonic measurement system 40 for convertingtemperature data into digital form for microprocessor 81.

In order to provide a representation of the distance D calculated inaccordance with travel time T of ultrasonic pulse 26, microprocessor 81generates a digital output signal on digital output bus 82. Digitaloutput bus 82 is coupled by way of optoisolator 86 to analog outputgenerator 88. Analog output generator 88 converts the digital signalfrom optoisolator 86 into an analog form. Analog output generator 88generates a four to twenty milliamp current output for use in standardindicator devices, controller devices and the like. In standardindicator devices of this type a four milliamp output can indicate, forexample, that storage vessel 12 is substantially empty. A twentymilliamp output can indicate that storage vessel 12 is substantiallyfull. Additionally, the output of analog output generator 88 can beapplied to a control device (not shown) for controlling the level Lmaterial 14 within storage vessel 12.

Microprocessor 81 also provides an output signal on output bus 84 torelay driver 90. Relay driver 90 can generate signals for driving relaysto indicate various alarm conditions associated with the measurementsperformed by ultrasonic measurement system 40. The conditions indicatedon output bus 84 can include material conditions exceeding alarm valuesdetermined, for example, by alarm switches 76. The conditions indicatedon output bus 84 can also indicate conditions below alarm values set byalarm switches 76. For example, the condition indicated on output bus 84can be that the level of material surface 18 is in the near zone whereinmeasurements may be less accurate.

The conditions indicated by relay driver 90 can also include a lost echocondition when no threshold-exceeding echo is detected by echo signalprocessor 58 within a predetermined time after application of crystalexcitation signal 83.

Power supply 92 receives input power and provides isolated supply 94 foranalog output generator 88. Power supply 92 also provides supply 96 tothe remainder of the circuitry of ultrasonic measurement system 40.

Referring now to FIGS. 3, 4, there are shown a block diagramrepresentation of ultrasonic measurement system 75 of the presentinvention including high discrimination ultrasonic measurement device 73as well as graphical representation 120. It is understood by thoseskilled in the art that ultrasonic pulse 26 is attenuated by severaleffects as it travels along its path from ultrasonic transducer 6,through air 16 to material surface 18, and back to transducer 6 afterreflection off material surface 18. In accordance with the method of thepresent invention ultrasonic measurement system 75 is adapted tocompensate for selected attenuation effects in order to provide improvedultrasonic measurements.

One effect that attenuates ultrasonic pulse 26 is a geometric effectwherein pulse 26 is attenuated due to the spreading of the acousticenergy of pulse 26 as pulse 26 travels through a medium. The geometriceffect is graphically illustrated by curve 128 of graphicalrepresentation 120. The attenuation caused by the geometric effect issubstantially an inverse square attenuation, although during the firstfew feet of pulse travel the pulse behaves as if it is collimated andthe attenuation can be somewhat more linear.

Another effect that attenuates ultrasonic pulse 26 is signal loss due toacoustic energy being absorbed by the medium as ultrasonic pulse 26passes through the medium. This effect is substantially logarithmic andcan be measured in dB/meter. This substantially logarithmic effect isrepresented as curve 124 of graphical representation 120. Thesubstantially logarithmic effect represented by curve 124 is dependentupon the humidity of the air when ultrasonic pulse 26 passes throughair. Increased humidity can result in increased attenuation up to a peakattenuation, followed by a decrease in attenuation as humidity increasesfurther. The attenuation caused by the substantially logarithmic effectis also dependent upon the frequency of ultrasonic pulse 26, whereinhigher frequencies are attenuated more than lower frequencies byhumidity. For example, the attenuation caused by the logarithmic effectcan peak at approximately sixty percent humidity for a fifty kilohertzultrasonic signal.

The result of the substantially inverse square effect and thesubstantially logarithmic effect operating simultaneously is an echosignal intensity that can diminish by a factor of several thousand asdistance D varies, for example, from one foot to forty feet, a desirablespan for ultrasonic measurement system 75 to measure. The measured valueof the voltage of the response of ultrasonic transducer 6 to thereflected ultrasonic signal can vary accordingly, as indicated by curve132 of graphical representation 120.

Thus, the amplitude of the electrical signal representing the echo pulseapplied to high discrimination ultrasonic measurement device 73 fallsoff as a function of distance D as illustrated by curve 132. Curve 132can be determined, for example, by emitting a plurality of ultrasonicpulses 26 for a plurality of differing distances D, preferably over theentire measurement range of storage vessel 12, and measuring, for eachpulse, the resulting pulse travel time and the corresponding amplitudeof the electrical signal of line 61 or amplitude of the ultrasonic echosignal.

In the method of the present invention measurement device 73 is adaptedto provide variable gain to the echo-representing electrical signal ofline 61 in order to linearize the electrical signal at the output ofdevice 73 over the entire range of distance D with respect to distanceD. The variable gain applied by measurement device 73 is selected tocause the amplitude at the output of measurement device 73 applied tooutput line 63 to be substantially constant with respect to distance D.

In order to cause the amplitude at the output of measurement device 73to be substantially constant with respect to distance D while theamplitude of the input follows curve 132, a numerical approximation ismade of curve 132. Signals representing the numerical approximation areapplied to the gain control of a variable gain device within highdiscrimination ultrasonic measurement device 73. In this mannermeasurement device 73 compensates for both the inverse square near zoneattenuation represented by curve 128 and the logarithmic attenuationrepresented by curve 124.

Since the logarithmic effect is dependent upon the relative humidity ofthe air 16 through which ultrasonic pulse 26 passes, the numericalapproximation is performed for a plurality of measurements performed ata selected humidity. In the preferred embodiment of the presentinvention the selected humidity can be thirty-five percent becausethirty-five percent can be an average humidity for measurements made inaccordance with the present invention. However, it will be understoodthat the method of the invention can be advantageously applied to anumerical approximation performed for any values of relative humidity.

Referring now to FIG. 5, there is shown a schematic representation ofhigh discrimination circuitry 77 which is a possible embodiment of highdiscrimination ultrasonic measurement system 71. Within highdiscrimination circuitry 77 the variable gain suitable for performingthe near zone quieting is controlled by integrators 204, 212 rather thanmicroprocessor 81. Integrator 204 receives a constant K1 as its input asshown in block 206. Because the input to integrator 204 is constant, theoutput of integrator 204, applied to first integrator output line 210,is proportional to time. Integrator 204 begins integrating when it isreset by reset timer 200. Reset timer 200 performs the operation ofresetting integrator 204 when it receives the pulse of electricalexcitation signal 83 by way of crystal excitation line 42 which iscoupled to reset timer 200. Thus the output of integrator 204 at time Tis approximately proportional to the elapsed time since the launching ofultrasonic pulse 26 from transducer 6.

Integrator 212 receives the time signal from integrator 204 on firstintegrator output line 210 and integrates it to provide an output onsecond integration output line 214. Because the input of integrator 212is proportional to time, the output on second integrator output line 214is proportional to time squared. Integrator 212 is also reset by resettimer 200 when the pulse of crystal excitation signal 83 appears oncrystal excitation line 42.

The time output of integrator 204 on first integrator output line 210and the time squared output of integrator 212 on second integratoroutput line 214 are applied to integrator output summer 220. The twointegrator outputs can be provided with constant gains K3 and K4 asshown in blocks 215, 217, respectively. A constant gain offset can alsobe provided by applying a constant gain K2 to integrator summer 220 asshown in block 216.

The output of integrator output summer 220 is applied by way of gaincontrol line 222 to amplifiers 224, 226. Amplifier 224 receives as itsinput the output of amplifier 60, by way of line 61. The output ofamplifier 60 on line 61 is the echo-representing signal provided byultrasonic crystal 50. The output of amplifier 224 is applied toamplifier 226 which provides an amplified output on line 63. The gainprovided by each amplifier 214, 226 can range from one to thirty. Line63 is coupled to amplifier 64 which applies its output signal todetector 68.

Because integrators 204, 212 are cascaded their summed output can berepresented as a quadratic polynomial. Because the summed output ofintegrators 204, 212 is applied to the gain control of two cascadedamplifiers 224, 226, the gain at the output of amplifier 226 can berepresented as a fourth order polynomial therefore having four breakpoints. The constants of the fourth order polynomial signal at theoutput of amplifier 226 can be selected by adjusting constants K1, K2,K3 and K4 that are applied to integrators 204, 212.

Ideally a polynomial of a order higher than a fourth order is desirablein order to improve the approximation of curve 132. However, a fourthorder polynomial is suitable for approximating curve 132 with acceptanceaccuracy and making the amplitude of the voltage at the output ofamplifier 64 substantially constant with respect to distance D. It willbe understood that polynomial approximations of any order other than thefourth order can be provided within the system of the present inventionprovided the approximation of curve 132 is suitable.

Referring now to FIGS. 6A, B, there is shown schematic representation250. Schematic representation 250 is a more detailed representation ofan embodiment of the system of the present invention including highdiscrimination ultrasonic measurement device 73. When crystal excitationpulse 83 on excitation line 42 is applied to transistor 45, transistor45 turns on permitting capacitor 47 to discharge. When capacitor 47discharges, transistor 49 is turned off causing gate transistor 43 toturn on.

When gate transistor 43 turns on, transistors 51 are turned on therebysupplying a reset to integrators 204, 212 of reset timer 200. The resetpermits amplifiers 53, 55 of integrators 204, 212 to begin integrating.The output of amplifier 59, which receives the output of amplifier 55 ofintegration 212 is the quadratic polynomial previously described forapproximating curve 132. The quadratic polynomial signal of amplifier 59rides on a DC bias, with respect to line 57. The bias with respect toline 57 can be approximately two volts. Amplifier 66 removes the biasfrom the quadratic polynomial signal.

Line 61 of schematic representation 250 carries an electrical signalrepresentative of the echo pulse of ultrasonic crystal 50 as previouslydescribed. The echo-representing signal of line 61 is applied to theinput of amplifier 224 high discrimination circuitry 77. The output ofamplifier 224 is applied to the input of amplifier 226. The gain ofamplifiers 224, 226 is controlled by the quadratic polynomial ofintegrators 204, 212 by way of lines 201, 203.

Referring now to FIG. 7, there is shown ultrasonic measurement system400 of the present invention. Echo signal processor 58 of ultrasonicmeasurement system 400 is provided with parametric signal processor 402and parameter generator 404 for making the sensitivity of system 400constant with respect to the travel time of ultrasonic pulse 26.Software functions are performed within parameter generator 404 ofmeasurement system 400 in order to provide parameters representative ofthe numerical approximation in accordance with curve 132 as previouslydescribed. The applying of the generated parameters by parametergenerator 404 is controlled by microprocessor 81 by way of crystalexcitation line 42.

Parametric system processor 402 receives the parameters generated inthis manner from parameter generator 404 by way of line 406. Theparameters generated by parameter generator 404, in accordance with thesignals of excitation line 42, and applied to parametric signalprocessor 402, are effective to approximate the relationship illustratedby curve 132 of graphical representation 120 in substantially the samemanner as disclosed with respect to ultrasonic measurement circuitry 77.Thus the amplitudes of the electrical signals representative of the echosignal of ultrasonic measurement instrument 400 are linearized withrespect to pulse travel time.

Referring now to FIG. 8, there is shown an alternate embodiment of echosignal processor 58. In this embodiment of echo signal processor 58,microprocessor 81 performs the software functions necessary to providethe numerical approximation in accordance with curve 132 as previouslydescribed. The parameters generated in this manner are provided bymicroprocessor 81 in digital form on parameter lines 110. The digitalparameters are applied to digital-to-analog converter 420 which convertsthem to analog form and applies them to parametric signal processor 402.

The analog parameters applied to parametric signal processor 402 bydigital-to-analog converter 420 are effective to approximate therelationship illustrated by curve 132 of graphical representation 120 insubstantially the same manner as disclosed with respect to circuitry 77.Thus the amplitudes of the electrical signals representative of the echosignals of an ultrasonic measuring system that includes the echo signalprocessor 58 of FIG. 8 can be made independent of the pulse travel timeor the distance D.

Referring now to FIG. 9, there is shown ultrasonic measurement system500 of the present invention. Ultrasonic measurement system 500 is analternate embodiment of ultrasonic measurement system 75 of the presentinvention. Two ultrasonic crystals 50a, b are provided in ultrasonicmeasurement system 500. Each ultrasonic crystal 50a, b of system 500 isincluded in a separate sensor 52a, b. Both ultrasonic crystals 50a, bare driven in response to microprocessor 81 by way of crystal excitationline 42 in the manner previously described with respect to ultrasonicmeasurement system 40. Furthermore, both ultrasonic crystals 50a, b aredriven simultaneously and in parallel with each other in response tomicroprocessor 81. Thus, high voltage excitation pulse 83 at the outputof transformer 46 is applied simultaneously to ultrasonic crystal 50a byway of diodes 48a and to ultrasonic crystal 50b by way of diodes 48b.

While crystals 58a, b are driven simultaneously within ultrasonicmeasurement system 500, the echo signals received by ultrasonic crystals50a, b are processed separately. In order to perform the separateprocessing of the echo signals received by ultrasonic crystals 50a, b,ultrasonic measurement system 500 is provided with echo processor switch122. Echo processor switch 122 operates under the control ofmicroprocessor 81 by way of switch control line 126 to alternately applythe output of ultrasonic crystals 50a, b to echo system processor 58.

Referring now to FIG. 10, there is shown ultrasonic measurement system550 of the present invention. Ultrasonic measurement system 550 isprovided with two sensors 52a, b, each having an ultrasonic crystal 50a,b. Ultrasonic crystals 50a, b of ultrasonic measurement system 550 arenot driven simultaneously by microprocessor 81. Rather, excitationsignals 83 from the secondary of transformer 46 of measurement system550 are alternately applied to ultrasonic crystals 50a, b by excitationswitch 124. Excitation switch 124 operates under the control ofmicroprocessor 81 by way of switch control line 128. The outputs ofcrystals 50a, b, of ultrasonic measurement system 550, representative ofecho signals within vessel 12, are applied to separate echo signalprocessors 58a, b.

Echo signal processor 58a receives the output of ultrasonic crystal 50a,by way of resistor 54a, clamped by diodes 56a. In accordance with thisinput, echo signal processor 58a provides a detect signal on detect line71a. The detect signal on detect line 71a is applied to microprocessor81.

Echo signal processor 58b receives the output of ultrasonic crystal 50b,by way of resistor 54b, clamped by diodes 56b. In accordance with thisinput, echo signal processor 58b provides a detect signal on detect line71b. The detect signal on detect line 71b is applied to microprocessor81.

Referring now to FIG. 11, there is shown traveling screen ultrasonicmeasurement system 600. In traveling screen ultrasonic measurementsystem 600 a traveling screen 606 is inserted into water flow 604 inorder to permit the water of water flow 604 to pass therethrough whilepreventing debris within the water from passing therethrough. Travelingscreen 606 is moved under the control of ultrasonic measurement system600 in order to remove debris which collects against traveling screen606. Traveling screen ultrasonic measurement system 600 can be used, forexample, at the water intake of a power plant or a water treatmentsystem.

When debris collects against traveling screen 606 on the upstream sideof traveling screen 606, water does not flow through traveling screen606 as well as it flows when debris is removed. The collected debriscauses the water level of upstream portion 610 of water flow 604 to riseto a higher level than the level of downstream portion 602. This causesa difference between the water levels of portions 610, 602 of water flow604. When the difference between the levels of portions 610, 602 reachesa predetermined magnitude, traveling screen 606 can be moved to removedebris and permit better water flow therethrough.

In order to detect when the difference between the two water levels hasreached the predetermined magnitude, two ultrasonic transducers 52a, bare provided in system 600. Ultrasonic transducer 52a detects distanceD2. Distance D2 represents the water level of downstream portion 602.Ultrasonic transducer 52b detects the distance D1 of water flow 604.Distance D1 represents the water level of upstream portion 604. Thedifference between the two levels can be determined by subtractingdistance D1 from distance D2.

It is understood by those skilled in the art that either ultrasonicmeasurement system 500 or ultrasonic measurement system 550 can beapplied to traveling screen ultrasonic measurement system 600. Whenultrasonic measurement system 500 is applied to measurement system 600,ultrasonic pulse 26 is applied to both downstream portion 602 of waterflow 604 and upstream portion 610 of water flow 604 on every measurementcycle. The output signal on line 28 of traveling wave ultrasonicmeasurement system 600 alternately represents the received signals ofsensors 52a, b on every other measurement cycle, as selected by echoprocessor switch 122. When ultrasonic measurement system 550 is appliedto measurement system 600, sensors 52a, b alternately emit ultrasonicpulse 26 under the control of excitation control switch 124.

Referring now to FIG. 12, there is shown ultrasonic measurement system650. Ultrasonic measurement system 650 is useful for compensating forchanges in material 16 resulting in changes in the velocity of pulse 26through material 16. In order to compensate for the effect of thevelocity of material 16 on measurements performed by system 650, sensor4b is provided on sidewall 30 of storage vessel 12. Additionally, sensor4a is provided on vessel bottom 24.

Distance D of ultrasonic measurement system 650 can be calculated as theknown reference distance from side to side in storage vessel 12, dividedby the measured reference time for ultrasonic pulse 27 to travel fromside to side of storage vessel 12, multiplied by the measured traveltime T of ultrasonic pulse 26. The measurement of distance D usingultrasonic pulse 26 is thus compensated for changes in material 14.Ultrasonic measurement system 500 or ultrasonic measurement system 550can be applied to ultrasonic measurement system 650. Ultrasonicmeasurement system 650 can be provided with a further ultrasonictransducer 4c in order to permit measurements to be taken from vesselbottom 24.

Referring now to FIG. 13, there is shown ultrasonic measurement system700. Using ultrasonic measurement system 700, it is possible to performdata communication with microprocessor 81 by way of the same line usedby system 700 for system output.

Port 705 of ultrasonic measurement system 700 can be used to provide thefour milliamp to twenty milliamp measurement output signal in accordancewith the method of the present invention as previously described.Additionally, data communication signals can be received and transmittedby way of port 705 as follows. The data communication signals areapplied by modulator 704 to filter 710. The signals can be, for example,pulse modulation width signals.

Detector 720, in cooperation with threshold 719, provides a squared upsignal 722 which is applied to HART 726 and, therefrom, to HART 730 in aconventional manner. The signal is then applied by UART 734 tomicroprocessor 81 by way of optoisolator 746. Signals frommicroprocessor 81, received by UART 734 by way of optoisolator 86, canbe shaped and modulated by wave shaper 722 and modulator 714,respectively, prior to being applied to port 705. This permits remoteinterrogation and programming of microprocessor 81 by way of themeasurement output line coupled to port 705.

Referring now to FIG. 14, there is shown ultrasonic measurement system750. Ultrasonic measurement system 750 is an alternate embodiment ofultrasonic measurement systems 75, 400, 500, 550 and 700 that issuitable for two-wire operation. Both the application of a conventionaldc voltage supply and the receiving of a measurement output signal canbe performed at port 705 of ultrasonic measurement system 750 in a knownmanner. The voltage applied to port 705 of system 750 can be, forexample, twenty-four volts. The measurement output signal at port 705during two-wire operation of ultrasonic measurement system 750 isdetermined in accordance with the high discrimination ultrasonicmeasurement device 73 of the present invention.

Referring now to FIGS. 15A, B there are shown curves 800, 810representative of response tone bursts from ultrasonic crystal 50 forillustrating a manner in which the system of the present inventionimproves measurement linearity. Curve 800 represents a tone burst with atarget one foot from transducer 6 and curve 810 represents a tone burstwith a target thirty feet away. The system of the present inventionimproves measurement accuracy by eliminating spurious responses, in themanner previously described. Additionally, however, a source ofnon-linearity in the prior art due to detector 68 triggering atdiffering points within response bursts is eliminated by the presentinvention.

It is well understood by those skilled in the art that the response toneburst received by detector 68 varies with time over a substantial numberof cycles as shown in curves 800, 810. Additionally, it is known thatdetector 68 triggers earlier in the curve for stronger responses andlater in the curve for weaker responses. Thus reflections from shorterdistances result in triggering earlier in the pulse and therefor causenon-linearity. For example, the trigger of curve 800 occurred at time820 and the trigger of curve 810 occurred at time 830, for a differenceof time 850. It is also understood that reflections from closer materialsurfaces 18 provide stronger responses.

In accordance with the present invention however the response signalsfrom material surface 18 closer to transducer 6 are substantially thesame strength as response signals from material signal 18 further away.Therefore, detector 68 triggers at the same point in the response curveregardless of distance D.

It will be understood that the frequency of the energy of ultrasonicpulse 26 of the material monitoring instruments of the present inventionis usually higher than the twenty kilohertz upper frequency limit ofhuman hearing. Therefore, such measurement instruments are typicallyreferred to as ultrasonic measurement instruments. However, it will beunderstood by those skilled in the art that the method of the presentinvention can be used by measurement instruments emitting energy of anyfrequency suitable for determining the monitored parameters.

We claim:
 1. An ultrasonic measurement method for use in an ultrasonicmeasurement system having an ultrasonic transducer, comprising the stepsof:(a) emitting an ultrasonic pulse from an ultrasonic transducer; (b)receiving an ultrasonic pulse in accordance with a pulse travel time,wherein the amplitude of said received ultrasonic pulse varies accordingto said pulse travel time; (c) providing a first electrical signalrepresentative of said received ultrasonic pulse; (d) applying variableamplification to said first electrical signal in accordance with saidpulse travel time; and (e) providing a second electrical signal inaccordance with said first electrical signal, wherein the amplitude ofsaid second electrical signal is substantially independent of said pulsetravel time during a time period corresponding to a range of said pulsetravel time.
 2. The ultrasonic measurement method of claim 1 comprisingthe step of determining said pulse travel time in accordance with ameasurement of the duration of a time interval.
 3. The ultrasonicmeasurement method of claim 2, comprising the step of measuring saidduration of said time interval by measuring the time elapsed betweensaid emitting and said receiving.
 4. The ultrasonic measurement methodof claim 3, comprising the step of determining a distance D inaccordance with said pulse travel time.
 5. The ultrasonic measurementmethod of claim 4, wherein the step of determining said distance Dcomprises determining said distance D in accordance with therelationship D=RT/2, wherein R is the speed of sound and T is saidelapsed time.
 6. The ultrasonic measurement method of claim 5, whereinsaid distance D is representative of the distance between saidtransducer and a material surface.
 7. The ultrasonic measurement methodof claim 6, comprising the step of determining a further distance inaccordance with said determined distance D.
 8. The ultrasonicmeasurement method of claim 7, wherein said further distance comprisesthe distance between a vessel bottom and said material surface.
 9. Theultrasonic measurement method of claim 1, comprising the step ofproviding a gain control signal in accordance with said pulse traveltime.
 10. The ultrasonic measurement method of claim 9, comprising thestep of applying said gain control signal to a variable amplifier. 11.The ultrasonic measurement method of claim 9, wherein said ultrasonicmeasurement system includes at least one integration device comprisingthe step of determining said gain control signal by integrating duringsaid pulse travel time using said integration device.
 12. The ultrasonicmeasurement method of claim 11, comprising the step of selectivelyapplying constants to said integration device to provide measurementsensitivity substantially independent of said pulse travel time.
 13. Theultrasonic measurement method of claim 9, comprising the step ofproviding a processor for providing said gain control signal.
 14. Theultrasonic measurement method of claim 13, comprising the step ofperforming calculations by said processor to provide said gain controlsignal.
 15. The ultrasonic measurement method of claim 13, comprisingthe step of performing table look ups by said processor to provide saidgain control signal.
 16. The ultrasonic measurement method of claim 1,comprising the step of applying a drive pulse to an ultrasonictransducer having an ultrasonic crystal for emitting said ultrasonicpulse.
 17. An ultrasonic measurement method, comprising the steps of:(a)emitting an ultrasonic pulse from an ultrasonic transducer; (b)receiving an ultrasonic pulse in accordance with a pulse travel time,wherein the amplitude of said received ultrasonic pulse varies accordingto said pulse travel time; (c) providing a first electrical signalrepresentative of said received ultrasonic pulse; (d) providing a gaincontrol signal in accordance with said pulse travel time; (e) applyingsaid gain control signal to a variable amplifier that amplifies thefirst electrical signal; (f) providing a second electrical signal inaccordance with said first electrical signal, wherein the amplitude ofsaid second electrical signal is substantially independent of said pulsetravel time during a time period corresponding to a range of the pulsetravel time; and (g) applying the output of said variable amplifier to afour to twenty amp indicator device.
 18. The ultrasonic measurementmethod of claim 1, wherein said received ultrasonic pulse is formed byreflecting said emitted ultrasonic pulse off an object.
 19. Theultrasonic measurement method of claim 18, wherein said object comprisesa material surface.
 20. An ultrasonic measurement method, comprising thesteps of:(a) emitting an ultrasonic pulse from an ultrasonic transducer:(b) receiving an ultrasonic pulse in accordance with a pulse traveltime, wherein the amplitude of said received ultrasonic pulse variesaccording to said pulse travel time; (c) providing a first electricalsignal representative of said received ultrasonic pulse; (d) providing again control signal in accordance with said pulse travel time; (e)applying said gain control signal to cascaded variable amplifiers thatamplify the first electrical signal; and (f) providing a secondelectrical signal in accordance with said first electrical signal,wherein the amplitude of said second electrical signal is substantiallyindependent of said pulse travel time during a time period correspondingto a range of the pulse travel time.
 21. The ultrasonic measurementmethod of claim 1, wherein said second electrical signal is applied toan analog-to-digital converter.
 22. The ultrasonic measurement method ofclaim 1, wherein said emitted ultrasonic pulse is attenuated to providesaid received ultrasonic pulse and step (d) compensates for saidattenuating.
 23. The ultrasonic measurement method of claim 1, whereinsaid method includes providing a plurality of ultrasonic transducerseach transducer of said plurality of transducers emitting an ultrasonicpulse to provide a plurality of ultrasonic pulses.
 24. The ultrasonicmeasurement method of claim 23, comprising the step of determining aplurality of distance measurements in accordance with said plurality ofultrasonic pulses.
 25. The ultrasonic measurement method of claim 24,comprising the step of controlling a traveling gate in accordance withsaid plurality of distance measurements.
 26. The ultrasonic measurementmethod of claim 24, comprising the step of determining a plurality oflevels of a material surface in accordance with said plurality ofdistance measurements.
 27. The ultrasonic measurement method of claim 1,comprising the step of emitting a plurality of ultrasonic pulses anddetermining the relationship between said pulse travel time and saidamplitude of said first electrical signal for each pulse of saidplurality of pulses.
 28. The ultrasonic measurement method of claim 27,comprising the step of approximating said relationship to provide aplurality of correction parameters.
 29. The ultrasonic measurementmethod of claim 28, wherein step (d) comprises the step of applying saidcorrection parameters to said first electrical signal.
 30. An ultrasonicmeasurement method, comprising the steps of:(a) emitting an ultrasonicpulse from an ultrasonic transducer; (b) receiving an ultrasonic pulsein accordance with a pulse travel time, wherein the amplitude of saidreceived pulse varies according to said pulse travel time; (c) providinga first electrical signal representative of said received ultrasonicpulse; (d) applying relatively lower amplification to said firstelectrical signal for relatively lower values of the pulse travel timeand applying relatively larger amplification to said first electricalsignal for relatively higher values of the pulse travel time; and (e)providing a second electrical signal in accordance with said firstelectrical signal, wherein the amplitude of the second electrical signalis substantially independent of said pulse travel time during a timeperiod corresponding to a range of the pulse travel time.
 31. Anultrasonic measurement method, comprising the steps of:(a) emitting anultrasonic pulse from an ultrasonic transducer; (b) receiving anultrasonic pulse in accordance with a pulse travel time, wherein theamplitude of said received ultrasonic pulse varies according to saidpulse travel time; (c) providing a first electrical signalrepresentative of said received ultrasonic pulse; (d) providing a gaincontrol signal for amplification of the first electrical signal inaccordance with said pulse travel time, wherein said gain control signalis determined by performing first and second integrations during saidpulse travel time to provide first and second integration signals; and(e) providing a second electrical signal in accordance with said firstelectrical signal, wherein the amplitude of said second electricalsignal is substantially independent of said pulse travel time during atime period corresponding to a range of the pulse travel time.
 32. Theultrasonic measurement method of claim 31, wherein said firstintegration signal is proportional to said pulse travel time and saidsecond integration signal is proportional to said pulse travel timesquared.
 33. The ultrasonic measurement method of claim 31, comprisingthe step of summing said first and second integration signals to formsaid gain control signal.
 34. An ultrasonic measurement method,comprising the steps of:(a) emitting an ultrasonic pulse from anultrasonic transducer; (b) receiving an ultrasonic pulse in accordancewith a pulse travel time, wherein the amplitude of said receivedultrasonic pulse varies according to said pulse travel time; (c)providing a first electrical signal representative of said receivedultrasonic pulse; (d) providing a second electrical signal in accordancewith said first electrical signal, wherein the amplitude of said secondelectrical signal is substantially independent of said pulse travel timeduring a time period corresponding to a range of the pulse travel time;wherein a level of a material is controlled in accordance with saidsecond electrical signal.
 35. An ultrasonic measurement method,comprising the steps of:(a) emitting an ultrasonic pulse from anultrasonic transducer; (b) receiving an ultrasonic pulse in accordancewith a pulse travel time, wherein the amplitude of said receivedultrasonic pulse varies according to said pulse travel time, and theemitted ultrasonic pulse is attenuated to provide said receivedelectronic pulse; (c) providing a first electrical signal representativeof said received ultrasonic pulse; and (d) providing a second electricalsignal in accordance with said first electrical signal, wherein theamplitude of said second electrical signal is substantially independentof said pulse travel time during a time period corresponding to a rangeof the pulse travel time; wherein step (d) compensates for saidattenuating and said compensating includes compensating for humidity.36. An ultrasonic measurement, comprising the steps of:(a) emitting anultrasonic pulse from an ultrasonic transducer; (b) receiving anultrasonic pulse in accordance with a pulse travel time, wherein theamplitude of said received ultrasonic pulse varies according to saidpulse travel time; (c) providing a first electrical signalrepresentative of said received ultrasonic pulse; (d) providing a secondelectrical signal in accordance with said first electrical signal,wherein the amplitude of said second electrical signal is substantiallyindependent of said pulse travel time during a time period correspondingto a range of said pulse travel time; and (e) applying said secondelectrical signal to a two-wire measurement system.
 37. An ultrasonicmeasurement method, comprising the steps of:(a) emitting an ultrasonicpulse from an ultrasonic transducer; (b) receiving an ultrasonic pulsein accordance with a pulse travel time, wherein the amplitude of saidreceived ultrasonic pulse varies according to said pulse travel time;(c) providing a first electrical signal representative of said receivedultrasonic pulse and applying variable amplification to said firstelectrical signal in accordance with said pulse travel time; (d)providing a second electrical signal in accordance with said firstelectrical signal, wherein the amplitude of said second electricalsignal is substantially independent of said pulse travel time during atime period corresponding to a range of the pulse travel time; and (e)determining the level of a material in accordance with said secondelectrical signal.
 38. An ultrasonic measurement system, comprising:(a)means for emitting an ultrasonic pulse from an ultrasonic transducer;(b) means for receiving an ultrasonic pulse in accordance with a pulsetravel time, wherein the amplitude of said received ultrasonic pulsevaries according to said pulse travel time; (c) means for providing afirst electrical signal representative of said received ultrasonic pulseand applying variable amplification to said first electrical signal inaccordance with said pulse travel time; (d) means for providing a secondelectrical signal in accordance with said first electrical signal,wherein the amplitude of said second electrical signal is substantiallyindependent of said pulse travel time during a time period correspondingto a range of the pulse travel time; and (e) means for determining thelevel of a material in accordance with said second electrical signal.39. An ultrasonic measurement system, comprising:(a) an ultrasonictransducer that emits an ultrasonic pulse; (b) a receiver that receivesan ultrasonic pulse in accordance with a pulse travel time, wherein theamplitude of said received ultrasonic pulse varies according to saidpulse travel time; (c) signal processing circuitry, coupled to thereceiver, that provides a first electrical signal representative of saidreceived ultrasonic pulse and a second electrical signal in accordancewith said first electrical signal, wherein the amplitude of said secondelectrical signal is substantially independent of said pulse travel timeduring a time period corresponding to a range of the pulse travel time;and (d) a microprocessor that determines the level of a material inaccordance with said second electrical signal.
 40. The system of claim39, wherein the receiver is integral with the ultrasonic transducer. 41.An ultrasonic measurement system having an ultrasonic transducer,comprising:(a) means for emitting an ultrasonic pulse from saidultrasonic transducer; (b) means for receiving an ultrasonic pulse inaccordance with a pulse travel time, wherein the amplitude of saidreceived ultrasonic pulse varies according to said pulse travel time;(c) signal processing circuitry that provides a first electrical signalrepresentative of said received ultrasonic pulse, applies variableamplification to said first electrical signal in accordance with saidpulse travel time, and provides a second electrical signal in accordancewith said first electrical signal, wherein the amplitude of said secondelectrical signal is substantially independent of said pulse travel timeduring a time period corresponding to a range of said pulse travel time.42. The system of claim 41, wherein the receiver is integral with theultrasonic transducer.